Transportation surfaces such as roads, runways, bridges and sign surfaces are generally marked using colored coatings such as paints. For example, roads, runways and bridges generally include lane markings, pedestrian crossings, directional markings and other markings which help to control the flow of traffic. Signs also include markings such as safety markings to provide information to the vehicle operator. Because these markings help to maintain order on roads, runways and other transportation surfaces, these markings should be highly visible to the driver under different weather conditions.
Heretofore, markings for transportation and sign surfaces have been produced using solvent-based organic binders. However, solvent-based organic coating materials are associated with the release of volatile organic compound (VOC) into the atmosphere. VOC is environmentally objectionable and is considered one of the major causes of the breakdown of the protective ozone layer of the earth.
Another problem with organic-based coatings is that these coatings tend to deteriorate during normal use. Specifically, the organic-based coatings degrade under ultraviolet light which greatly shortens the service life of the markings.
One alternative to solvent-based organic binders are coating compositions which use thermoplastic binders. Thermoplastic binders are not solvent-based materials and thus do not release VOC. Therefore, thermoplastic binders share a large portion of the market for transportation marking paints. Nevertheless, the application of paints having thermoplastic binders is rather difficult because it requires heating and spreading the paints during application of the paints. Furthermore, it is hard to clean painting machinery and equipment which has been used with paints containing thermoplastic binders. Moreover, the range of thicknesses for which these paints can be applied is limited.
One alternative to overcome the problems of organic-based coatings has been the use of coatings which use inorganic binders. For example, U.S. Pat. No. 3,977,888 to Sano et al. describes preparing a colored inorganic coating material having excellent adherence, high crack resistance, fire resistance, weather resistance, and high gloss. The colored inorganic coating material is formed by applying an aqueous, viscous solution of an alkali silicate and pigments and/or fillers to a substrate, and applying a hardener to the inorganic coating material to harden the inorganic coating material. U.S. Pat. No. 5,164,003 to Bosco et al. describes a coating material for the protection of substrates from wear and corrosion which comprises a silica-based binder of an alkali metal silicate, silica gel, a crosslinking agent and water. Nevertheless, these inorganic-based materials do not possess the desired properties for transportation and sign surfaces such as retroreflectivity and flexibility and are therefore not fit for this purpose.
An additional concern in the application of coatings for marking transportation and sign surfaces is the accumulation of dirt on the surface markings. The accumulation of dirt causes a reduction in the visibility of the surface marking during daylight and the retroreflectivity of the surface marking at night. Unfortunately, however, conventional coatings generally attract dirt from the environment and ongoing traffic.